When an annular workpiece is to be machined in a mechanical treatment machine, the workpiece is usually clamped on a disk-shaped rotating, magnetic chuck. The mechanical mechanism used for feeding the workpieces onto the chuck, commonly robots, ordinarily have a rather low precision. Thus, the workpiece is often placed in a position where its center does not coincide with the rotational center of the magnetic chuck.
For obtaining a better coincidence between the center of the chuck and the annular workpiece, when this is positioned thereon, it is of course possible to use high precision robots, with intricate sensing and positioning systems. However, this entails quite expensive equipment.
The annular blanks are furthermore seldom perfectly round, but more or less oval, which also makes it difficult to have the ring members positioned so that they can be readily machined when affixed on the rotating magnetic chuck, even when high precision feeding members are used.
JP 10-043985 discloses a centering method incorporating a rotary magnetic chuck with a circular workpiece positioned thereon, and with driving parts arranged to move towards the center of the chuck. Measuring instruments move simultaneously to a centering measuring reference surface and send a signal when the measured value falls within a prescribed range, whereupon the driving parts are returned to their prescribed position. During this time, the measuring instruments continue measurement and when the numeric value does not change, a grinding process is started. This method is referred to as “automatic”, but it thus utilizes measuring instruments for controlling and monitoring the centering.